The invention applies to a system for braking the moving elements of a focal plane shutter for a photographic camera, with the braking system becoming effective just before the moving elements reach their stop or end position. Such focal plane shutters usually have thin and lightweight metal blades that are combined in an overlapping arrangement to form a shutter curtain, and the shutter can include one element that moves to open the shutter and another element that moves to close the shutter. Such moving shutter parts require considerable spring force in spite of their relatively small mass and inertia, and the spring forces have to be effective over the entire extent of the picture window throughout the entire distance of travel of the shutter elements to make the slot speed exact throughout its distance of travel. This requires braking of a considerable amount of energy as the shutter elements approach their stop positions, and the braking force is relatively abrupt in stopping the shutter elements.
Brake systems for abrupt stopping of shutter elements moving at the focal plane are generally known. Among others, it is already shown in German Pat. No. 1,090,508, issued Mar. 23, 1961, to brake the moving elements of a focal plane shutter by a belt brake that is similar to a drum brake. Such a belt brake applies a predetermined brake torque to a brake drum that is operatively connected with shutter curtain parts in a coupling that includes a coil spring, an engaging drum, and a forked lever. The coupling mechanism only operates in one direction, as a known coil spring coupling and transmits brake torque of the drum brake to the shutter curtain during braking operations and then uncouples the brake spring of the drum brake to prevent unreasonably high shutter cocking forces. Such an arrangement requires relatively complex coupling devices, and its coupling coil spring is subject to variations in slip that are changed by external influences so that it is not completely reliable and also occupies considerable space.
The invention involves recognition of the problems of prior art braking systems for focal plane seutters and a realization of a way of providing a braking system that operates reliably and effectively and also gently and smoothly in braking the moving elements of a focal plane shutter without causing any bumpings or vibrations. The invention also combines these virtues with a compact brake system that requires little space, is simple, efficient, and effective, and provides a braking torque that is conveniently adjustable and capable of accurately establishing the end or stop positions of the shutter elements (blade overstroke).